I think upper 70's would be ideal. I have read that tads kept in cooler temps tend to take almost twice as long to morph...although they end up morphing out a little larger.
I have also heard tads kept in too high of temps run a high risk of development defects.
I keep my Auratus tads room temp (68-74 about) and have had no problems or defects so far. They have been morphing out in about 3 months.